Field of Invention
The present invention relates to a method for producing zinc oxide, and more particularly to a method for producing high-purity nanometer zinc oxide.
Description of Related Arts
At present, the raw material for producing zinc oxide product usually is zinc calcine which calcined by ore with high content of zinc. The content of zinc is higher, so it is easy for leaching relatively. With years of mining, high-grade ore source has become less and less, and the grade of ore is gradually reduced, people now begin to pay attention to the use of zinc oxide ore. But the content of zinc in zinc oxide ore is usually low (content of valuable elements: Zn below 30%; Pb 0.15%; Cu 0.008%; Mn 0.06%), and its composition is complicated, it exist in the form of smithsonite, zinc ferrites ore or hemimorphite, and with high content components of iron oxide, silicon oxide, calcium oxide and magnesium oxide (respectively Fe2O3 4-7%; SiO2 3-5%; CaO 30-32%; MgO 7-8%), mud content is high. The value of comprehensive recovery and utilization is little. Ore dressing and acid leaching of zinc are more difficult, the Ore dressing cost is high, it is the major technical problems in the filed of Ore dressing at home and abroad long existing. In recent years, the leaching method for zinc oxide ore is leaching zinc oxide by zinc sulfate solution containing strong acid, although the zinc leached rate has improved, but the content of iron and silicon entering into solution is also high, iron removal is difficult, reagent consumption amount is large, the washing liquid take away many zinc. China public patent such as CN1477217A has improved the above method. Firstly, perform neutral leaching adopting the solution containing zinc sulfate and pH=3-4, and then perform low acid leaching process. But the zinc ferrites ore and hemimorphite decompose slowly at low acid conditions, and it still exist the issues of low leaching efficiency, high cost, environmental pollution and other issues. Many metallurgical workers at home and abroad all consider that the zinc content of less than 20% Zinc Oxide mine should not be used alone to deal with wet leaching process.
The most ideal processing method for the low-grade zinc oxide ore is the selective leaching of zinc, it make the zinc entering into the final leached solution, and recycle zinc valuably.
High-purity zinc oxide usually refers to the zinc oxide product with the mass percent ≧99.7%. The high-purity zinc oxide is an indispensable raw material for the modern high technologies, with wide applications. It is mainly used in glass, feed, ceramics, dyes, paint, paper-making, rubber, pesticides, oil refining, galvanization, special steel, alloy, defence-related science and technology, etc. The glass, paper-making, or rubber, oil refining enterprises have high demands for zinc oxide and very high purity requirement.
Nanometer zinc oxide (ZnO) is a new type of high-function fine inorganic product with the particle size between 1 and 100 nm in the 21st century, exhibiting a variety of special properties such as non-migratory, fluorescence, piezoelectric, absorption and UV scattering ability, etc. With its special optical, electrical, magnetic and sensitivity performance, it can be used to produce gas sensors, phosphors, rheostat, UV shielding materials, image recording materials, piezoelectric materials, varistors, efficient catalysts, magnetic materials, and plastic films, etc.
Currently the methods producing zinc oxide mainly include chemical precipitation method, sol-gel method, microemulsion method and hydrothermal synthesis method, etc. But all raw materials used are zinc calcine or pure zinc salts (such as zinc sulfate, zinc nitrate or zinc acetate) with the zinc content higher than 50%.
Ammonia method is a commonly used method for producing zinc oxide. Currently, the ammonia method (ammonia-ammonium bicarbonate combined leaching method) for producing zinc oxide generally includes the following steps: leaching of zinc-containing materials using ammonia-ammonium bicarbonate as leaching agent, and after purification, ammonia evaporation crystallization, drying, calcinations of zinc-ammine complexing solution, produce the zinc oxide product.
Above-mentioned traditional ammonia method for producing zinc oxide has not been used in low-grade zinc oxide ore, mainly due to the following reasons:
1. Those ore have low zinc content and high mud content, the leached solution has low zinc concentration and high cost, so enterprises can not afford;
2. Because of complicated impurities, qualified rate of products is low, product prices are low and economic benefit is difference;
3. Conventional means of leaching, leached rate of zinc ore is low, residual zinc in leached residue is high, so it is high waste, zinc ore value not use and reflect.
Currently, the disclosed technologies of producing nanometer zinc oxide by ammonia leaching method are low-temperature hydrolysis methods, for example:
Chinese Patent Application No. 92103230.7 discloses an improved technology for producing zinc oxide aiming at traditional ammonia complexometry. The purified zinc-ammonia complexing solution is diluted with water to hydrolyze part of zinc-ammonia complexing solution and obtain the basic zinc carbonate (with a ratio of zinc hydroxide and zinc carbonate of 2:1), and then continue to heat until zinc-ammonia complexing solution is decomposed completely. After high-temperature calcinations, 30-100 nn nanometer zinc oxide is obtained.
For the patented technology, the following problems need to be solved:
After hydrolysis, in the thermal decomposition process of undissociated zinc-ammonia complexing solution, the newly generated basic zinc carbonate will continue to grow on the original surface of crystal nucleus, promoting the growing of originally hydrolyzed crystal, which is prone to cause uneven zinc carbonate crystal, making it difficult to control the particle size of the final product.It adds 4-10 times of water, reducing the efficiency in the preparation process, increasing the energy consumption and the water treatment cost at the back end.
Chinese Patent Application No. 200610130477.7 disclosed an improved technology producing zinc oxide for the traditional ammonia complex method. The zinc-ammonia complexing solution is mixed with 1:2-20 hot water or hot mother liquor continuously. After heating and heat preservation, it is recycled to be used in hydrolysis of zinc-ammonia complexing solution, to prepare 10-50 nn of nanometer zinc oxide.
For the patented technology, the following problems need to be solved:
After hydrolysis of mother liquor, the ammonia cannot be fully separated, and it cannot achieve the hydrolysis effect, and finally the zinc-ammonia complexing solution is mixed with the zinc-ammonia complexing solution.
For the above two patents, the nanometer crystals are obtained by slightly changing pH value with a large amount of water. In fact, relying solely on the pH value slight change, only a very small part of hydrolysis can be achieved (checked from the ammonium hydroxide solubility curve of zinc oxide). The higher concentration of zinc ammonia liquid, the higher the precipitation efficiency and lower energy consumption; while the lower concentration of zinc ammonia liquid, the lower the precipitation efficiency and high energy consumption. It is technically feasible by artificially increasing the proportion of water to produce nanometer zinc oxide, but it is not feasible in terms of economic efficiency.
In addition, for the current ammonia leaching method for producing zinc oxide, the crystal is basic zinc carbonate, with high decomposition temperature (the initial temperature of zinc hydroxide decomposition is about 125° C., and that of zinc carbonate is about 300° C.). In order to obtain high-purity products, it is necessary to guarantee a high decomposition temperature, generally controlled at above 500° C., to completely decompose the basic zinc carbonate. For example, in the Chinese Patent with Application No. 200610130477.7, the calcinations temperature is as high as 550° C. High-temperature calcinations seriously affect the specific surface area and dispersity of zinc oxide, and thereby affecting its application field.
In summary, for treatment process of the low-grade zinc oxide ore, how to effectively leach the zinc from materials with low zinc content and get high-purity nanometer zinc oxide and to overcome the disadvantages of traditional pyrometallurgy and hydrometallurgy have become technical problems urgently to be resolved in the industry.